Steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance, method of manufacturing the same, and mehod of manufacturing bolt using the same

ABSTRACT

Provided are a high-strength, high-manganese steel wire rod having excellent cold heading quality and not requiring spheroidizing and quenching-tempering treatments during manufacturing a bolt and a method of manufacturing a bolt using the steel wire rod. The method of manufacturing a steel wire rod includes heating a steel containing 12 to 25 wt % of Mn within a temperature range of 1100° C. to 1250° C., hot rolling the heated steel within a temperature range of 700° C. to 1100° C., and cooling the hot rolled steel to a temperature of 200° C. or less and cold caliber rolling or drawing to manufacture a steel wire rod.

CROSS REFERENCE

This application claims foreign priority under Paris Convention and 35U.S.C. §119 to Korean Patent Application Nos. 10-2010-0116350, filed 22Nov. 2010, and 10-2011-0022061 filed 11 Mar. 2011, with the KoreanIntellectual Property Office.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a steel wire rod used for a bolt, andmore particularly, to a high-strength, high-manganese steel wire rod,which has excellent cold heading quality and hydrogen delayed fractureresistance and is enabled to skip spheriodizing and quenching-temperingheat treatment processes, a method of manufacturing the same, and amethod of manufacturing a bolt using the same.

2. Description of the Related Art

There is a trend towards continuously increased demand for ahigh-strength wire rod in order to achieve energy efficiency increaseand energy consumption decrease. For example, when bolts used in anautomobile engine are replaced by high-strength bolts to reduce theweight of bolts by 80 g, a ripple effect on the associated components,i.e., 20 kg of engine weight reduction, will be very large. Therefore,continuous research and development on the application of high-strengthsteel for a component material have been required.

However, an obstacle in manufacturing of high-strength bolt steel isthat cold heading quality deteriorates due to the use of high carbonsteel for high strengthening. With respect to the high-strength steelwire rod having poor cold heading quality, life time of cold headingdies is decreased and this will cause an increase in production costs.

As a method for resolving the foregoing limitation, typical temperedmartensitic steels are used as a material for a high-strength wire rod.However, with respect to the foregoing martensitic steels, postprocessing has to be performed, in which cold heading is performed afterbeing subjected to a spheriodizing heat treatment requiring for about 20hours or more to obtain low flow stress in order to improve cold headingquality, and thereafter, quenching and tempering heat treatments areperformed to increase strength.

The spheroidizing heat treatment and the quenching and temperingprocesses before and after the cold heading cause cost increase andgenerate distortion during heat treatments with respect to a large bolt.In order to prevent the foregoing, various types of research have beenrecently conducted for manufacturing a heat treatment free high-strengthbolt.

However, with respect to a general heat treatment free bolt developed todate, cold cold heading quality is excellent but high strength has notbeen achieved. When a high-strength steel wire rod is used, materialstability was not guaranteed due to the generation of defects in a bolthead portion and the existence of residual stress, according to coldheading of the high-strength wire rod having poor cold heading quality.

Meanwhile, twinning induced plasticity (TWIP) steel, which ishigh-manganese steel, relieves strain energy by using twins. Themovement of dislocations is easier in comparison to a body centeredcubic (BCC) structure because many slip systems exist in the TWIP steeldue to the characteristics of its face centered cubic (FCC) crystalstructure. Since the deformation of the TWIP steel is accomplished bymeans of twins and dislocations, cold heading quality of the TWIP steelis very good and a strengthening effect is dynamically induced by usingtwins generated during deformation. Such nano-sized twins maximize agrain refinement effect and thus, a high-strength material may beobtained.

Korean Patent Application Laid-Open Publication No. 0851158 discloses arelated art with respect to the foregoing high-manganese steel. Theforegoing patent is related to a high-manganese, high-strength steelplate having excellent impact properties and a method of manufacturingthe same, and describes about a method of manufacturing a plate by usinghigh-manganese steel containing 10 wt % to 25 wt % of manganese (Mn).This technique may be applied to a plate, but the technique isinappropriate for manufacturing a wire rod through a caliber rollingmill or drawing which has a different deformation mode.

Also, the biggest obstacle in strengthening of a wired rod is hydrogendelayed fracture that deteriorates material stability. The hydrogendelayed fracture is generated by the reduction of the cohesive strengthof a material in such a manner that hydrogen atoms from hydrogen sulfidegas or water existing in an external environment penetrate into thematerial to weaken metallic bonds of the material. Since the hydrogensolubility of steel is very low at room temperature, the hydrogen atomspenetrated into the material exist by being trapped in dislocations,grain boundaries, and interphase boundaries which are energeticallystable. As a result, intergranular fracture is generated due to theintensive weakening of bond strength by means of hydrogen at grainboundaries or the like.

In particular, it has been known that hydrogen delayed fractureresistance significantly decreases in high-strength steel having astrength of 1 GPa or more. The reason for this is that diffusiblehydrogen trapping sites are increased according to the increase in thedensity of dislocation as an essential defect in a material and theincrease in grain boundary density due to the grain refinement as thematerial is subjected to high strengthening. Therefore, a wire rodhaving excellent hydrogen delayed fracture resistance as well as thedevelopment of high-strength steel is required.

SUMMARY OF THE INVENTION

The present invention addresses the above-identified, and other problemsassociated with conventional methods and apparatuses.

An aspect of the invention provides a steel wire rod promoting highstrengthening of the steel wire rod as well as having excellent coldheading quality and being enabled to skip spheriodizing andquenching-tempering heat treatment processes, and a method ofmanufacturing the same.

Another aspect of the invention provides a method of manufacturing abolt using the steel wire rod.

Another aspect of the invention provides a steel wire rod promoting highstrengthening as well as having excellent hydrogen delayed fractureresistance and a method of manufacturing the same.

According to an embodiment of the invention, there is provided a methodof manufacturing a steel wire rod having excellent cold heading qualityand hydrogen delayed fracture resistance including: heating a steelcontaining about 12 wt % to about 25 wt % of manganese (Mn) within atemperature range of about 1100° C. to about 1250° C.; hot rolling theheated steel within a temperature range of about 700° C. to about 1100°C.; and cooling the hot rolled steel to a temperature of 200° C. or lessand cold caliber rolling or drawing to manufacture a steel wire rod.

In the method of manufacturing a steel wire rod according to theinvention, the steel may include about 12 wt % to about 25 wt % ofmanganese (Mn), about 0.5 wt % to about 1.0 wt % of carbon (C), about1.0 wt % to about 2.0 wt % of aluminum (Al), residual iron (Fe), andunavoidable impurities.

Also, in the method of manufacturing a steel wire rod according to theinvention, the cold caliber rolling or drawing may be performed at areduction of area range of about 10% to about 70% and for example, thecold caliber rolling may be performed at a reduction of area range ofabout 30% to about 90%.

According to another embodiment of the invention, there is provided asteel wire rod having excellent cold heading quality and hydrogendelayed fracture resistance including about 12 wt % to about 25 wt % ofmanganese (Mn), wherein a microstructure of the steel wire rod includesa face centered cubic austenitic structure and a <112>{111} twin system.

In the method of manufacturing a steel wire rod according to theinvention, the steel wire rod includes a total of twelve twin systems infour <112> orientations and three {111} planes of a lattice and fourtwin variants are formed in one plane.

In the method of manufacturing a steel wire rod according to theinvention, a composition of the steel wire rod may include about 12 wt %to about 25 wt % of manganese (Mn), about 0.5 wt % to about 1.0 wt % ofcarbon (C), about 1.0 wt % to about 2.0 wt % of aluminum (Al), residualiron (Fe), and unavoidable impurities.

According to another embodiment of the invention, there is provided amethod of manufacturing a bolt including: cold heading a steel wire rodmanufactured by the method of manufacturing a steel wire rod accordingto the embodiment of the invention to manufacture a bolt head; andperforming a heat treatment on the cold headed steel wire rod within atemperature range of about 400° C. to about 600° C.

In the method of manufacturing a bolt according to the invention, theheat treatment may be performed for about 10 minutes or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating a manufacturing history of a steel wirerod according to an embodiment of the invention;

FIGS. 2( a) and 2(b) are a tensile curve and an optical micrograph ofinitial hot rolled steel, respectively;

FIGS. 3( a) to 3(d) illustrate image maps of back scattered electrondiffraction patterns of hot rolled steel and cold caliber rolled steelwire rods having a reduction of area of 31%, 43%, and 54%, respectively;

FIGS. 4( a) to 4(d) are true stress-true strain curves illustratingtension and compression measured from hot rolled steel and cold caliberrolled steel wire rods having a reduction of area of 31%, 54%, and 82%,respectively;

FIGS. 5( a) to 5(c) are tensile curves after heat treating cold caliberrolled steel wire rods having a reduction of area of 31%, 43%, and 54%at 550° C. and 600° C. for 8 minutes, respectively;

FIGS. 6( a) and 6(b) illustrate an image map and a grain map of backscattered electron diffraction patterns of Comparative Example, and 6(c)and 6(d) illustrate an image map and a grain map of back scatteredelectron diffraction patterns of Inventive Example 3, respectively;

FIGS. 7( a) and 7(b) illustrate transmission electron micrographs of aplate rolled steel sheet and a caliber rolled wire rod, respectively;

FIG. 8 is a graph illustrating changes in strength with respect todeformation according to a reduction of area during caliber rolling; and

FIG. 9 is a graph illustrating notch fracture strength according to adiffusible hydrogen content.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, the invention is described in detail.

The present inventors developed a high-strength steel wire rod havingexcellent cold heading quality by using a dynamic strengtheningdeformation mechanism of twinning induced plasticity (TWIP) steel. TheTWIP steel has excellent elongation and is enabled to omit heattreatments even though it is high-strength steel. It may be understoodthat the TWP steel had infinite strain without cracks according to theresults of a compression test performed on 1800 MPa class high-strengthsteel, which had been pre-deformed by indirect multi-axial tension usingcaliber rolling.

The reason for this is that when compressive stress is exerted on aspecimen under applied tensile stress, low yield strength is obtainedaccording to the characteristics of the TWP steel having high backstress known as the Bauschinger effect, and continuous deformation andhigh strengthening are achieved by means of a twin mechanism.

Also, in a body centered cubic crystal structure, hydrogen diffusionoccurs from an octahedral interstial site to a nearest octahedralinterstial site and thus, hydrogen delayed fracture occurs. In contrast,in a face centered cubic crystal structure, stable hydrogen at atetrahedral interstial site diffuses to a nearest tetrahedral interstialsite through an octahedral interstial site and thus, hydrogen diffusionin the face centered cubic crystal structure will be about 100 to 1000times slower than that of the body centered cubic crystal structure.Further, since hydrogen cracking of steel is a very low value of about10⁻⁴ ppm at room temperature, hydrogen penetrated into the steel mainlyexists by being trapped at microstructural defects such as dislocationsand grain boundaries. A typical steel wire rod secures strength throughhigh dislocation density and fine grains such that it is vulnerable tohydrogen trap. However, if the strength is secured by means of high twinboundary density instead of increasing dislocation density, hydrogendelayed fracture resistance may be improved because the density ofdiffusible hydrogen trap sites decreases.

The present inventors have developed a steel wire rod having excellentcold heading quality and hydrogen delayed fracture resistance based onthe observations, in which excellent cold heading quality may beobtained through the twin mechanism and Bauschinger effect, and hydrogendelayed fracture resistance may be improved when an austenitic singlephase having a face centered cubic structure with a low hydrogendiffusion rate and a deformation structure having mainly twins areincluded.

First, a method of manufacturing a steel wire rod according to theinvention is described in detail.

According to the method of manufacturing a steel wire rod of theinvention, steel containing 12 wt % to 25 wt % of manganese (Mn) isfirst heated within a temperature range of 1100° C. to 1250° C. Althougha composition of steel except manganese is not particularly limited, thecomposition of the steel may include 0.5 wt % to 1.0 wt % of carbon (C),1.0 wt % to 2.0 wt % of aluminum (Al), residual iron (Fe), andunavoidable impurities. However, the addition of other components inaddition to the foregoing composition is not excluded.

The heating of the steel within a temperature range of 1100° C. to 1250°C. is for a homogenization treatment of the steel, which is forpreventing segregation of elements occurred during casting, and forexample, the homogenization treatment may be performed within atemperature range of 1150° C. to 1200° C.

The heated steel is hot rolled within a temperature range of 700° C. to1100° C. The foregoing hot rolling corresponds to the hot rolling of awire rod for size adjustment. The hot rolling may be performed within atemperature range of 700° C. to 1100° C. because a decrease in areduction rate may occur during cold caliber rolling due to the twinsgenerated during the rolling when the hot rolling temperature is lessthan 700° C. and the decrease in the reduction rate occurs when thetemperature is more than 1100° C. due to the fact that grain sizebecomes so large that twins are not effectively generated during coldcaliber rolling.

The hot rolled steel is cooled to a temperature of 200° C. or less andthen a wire rod is manufactured by performing cold caliber rolling ordrawing. The reason for this is that the reduction of area of a materialrather decreases more due to the limitations in that generation of twinsis rapid at low strain and the twins are not generated at high strainwhen the cold caliber rolling temperature is more than 200° C. Thereduction of area during the caliber rolling or drawing varies accordingto the required strength of the wire rod. The reduction of area may be10% or more in order to increase yield strength and the reduction ofarea of 90% may be regarded as a possible limit for cold heading.Therefore, the caliber rolling or drawing may be performed within areduction of area range of 10% to 90% and for example, the reduction ofarea with respect to cold caliber rolling may be in a range of 30% to60% according to the use of a wire rod. Also, the lower limit of thetemperature is not particularly limited and room temperature may beused.

The strengthening effect of a material same as the grain refinement isobtained during the cold caliber rolling due to the generation of twinsin the material. Also, that cold reduction of area and strengtheninghigher than those of plate rolling may be obtained may be explainedthrough transmission electron micrographs of a plate rolled steel sheetand a caliber rolled wire rod as shown in FIG. 7. As shown in FIG. 7(b), different from FIG. 7( a) in which the plate rolling is performed,all four twin variants are generated during caliber rolling, which isdifferent from the fact that only a single twin variant is generatedduring typical plate rolling. According to the formation of the fourtwin variants, a grain refinement effect is maximized such that theachievement of high strength is facilitated, and the twins in fourdifferent orientations act as a main deformation mechanism in a materialand thus, high reduction of area may be obtained. Therefore,manufacturing of a steel wire rod having various strengths isfacilitated.

Also, since the hydrogen cracking of steel has a very low value of about10⁻⁴ ppm at room temperature, hydrogen penetrated into the steel mainlyexists by being trapped at microstructural defects such as dislocationsand grain boundaries. A typical steel wire rod secures strength throughhigh dislocation density and fine grains such that it is vulnerable tohydrogen trap. However, a high-manganese steel wire rod according to theinvention secures strength by low dislocation density and high twinboundary density based on dynamic strengthening effects as well asimproving hydrogen delayed fracture resistance due to the existence oflow density diffusible hydrogen trap sites.

Hereinafter, a steel wire rod according to the invention is described indetail.

The steel wire rod of the invention includes 12 wt % to 25 wt % of Mn. Acomposition of the steel wire rod of the invention may include 0.5 wt %to 1.0 wt % of carbon (C), 1.0 wt % to 2.0 wt % of aluminum (Al),residual iron (Fe), and unavoidable impurities.

The steel wire rod of the invention has high strength and highelongation because twins and dislocations act as a deformation mechanismat the same time during deformation. Also, a fine grain strengtheningeffect is generated by nano-sized twins induced during deformation aswell as increasing elongation by relieving strain energy. Low yieldstrength, which is a limitation of the hot rolled TWIP steel, may beincreased through cold caliber rolling or drawing. High dislocationdensity generated during cold deformation increases yield strength of amaterial. Further, with respect to the cold deformed steel, cold headingquality for a subsequent process is also excellent according to adeformation mechanism by means of twins.

The steel wire rod of the invention is austenitic steel having a facecentered cubic structure and has a <112>{111} twin system. A total oftwelve twin systems in four <112> orientations and three {111} planesare included in a lattice and at this time, formation of four twinvariants is possible in one plane. The main strengthening mechanism isan interaction between the four twin variants and dislocations. Aparticular point is that all the dislocations interacting with the twinsof the steel wire rod are dislocations in a tensile direction. When thedeformation in a direct tensile or an indirect tensile direction isapplied to the steel wire rod, dislocations only in a tensile directionare formed and these dislocations contribute to the improvement of coldforgeablity later. That is, flow stress of a material is lowered throughoffsetting the dislocations in a tensile direction with dislocations ina compression direction during compressive deformation, and thus, coldheading is facilitated.

Also, the high-manganese steel wire rod according to the invention has asingle phase austenitic structure. Since the austenitic steel has a facecentered cubic (FCC) structure having a low hydrogen diffusion rate,mobility of hydrogen trapped in the austenitic steel is lower than thatof typical ferrite steel. Further, the high-manganese steel, in whichdynamic strengthening through mechanical twins is achieved instead ofstrengthening by grain boundaries or ultra-fine carbides acting asdiffusible hydrogen trap sites, has a rather positive effect on hydrogendelayed fracture because twins act as the trap sites of non-diffusiblehydrogen. The reason for this is that trap activation energy withrespect to hydrogen is large because the mechanically formed twins arenot previously known commensurate interfaces but are non-commensurateinterfaces. Therefore, the high-manganese steel wire rod has very goodhydrogen delayed fracture resistance because it may have both of lowhydrogen diffusion rate and high hydrogen trap activation energy.

Hereinafter, a method of manufacturing a bolt using a steel wire rodaccording to the invention is described in detail.

In the manufacturing of the bolt using the steel wire rod according tothe invention, cold heading is performed to form a bolt head portion. Ina typical case, a spheroidizing heat treatment is performed in order toimprove cold heading quality, but cold heading may be directly performedby omitting the spheroidizing heat treatment in the invention. Sinceelongation is very low with respect to a typical high-strength bolt, alow-strength spheroidized material is cold forged and thereafter, ahigh-strength quenching/tempering (Q/T) heat treatment has to beperformed. However, the shape of the material may be distorted when theforegoing Q/T heat treatment is performed and economy deterioratedbecause the spheroidizing heat treatment takes about 20 hours.

The cold heading is performed, and then a stress-relief heat treatmentis performed within a temperature range of 400° C. to 600° C. Thestress-relief heat treatment may be performed within a temperature rangeof 400° C. to 600° C. in which recrystallization does not occur and theheat treatment time may not be more than 10 minutes. Heating may beperformed at a temperature of 400° C. or more because redistribution ofdislocations does not occur at a temperature of less than 400° C. Thestress-relief heat treatment redistributes tangled dislocations torelieve residual stress, and thus, contributes to increase the stabilityof a material by removing local stress concentration sites.

The invention has an advantage in comparison to the related art in thatseparate quenching and tempering are not performed after cold heading isperformed.

Hereinafter, characteristics of a steel wire rod manufactured accordingto an embodiment of the invention are described in detail.

Steel wire rods were manufactured by performing cold caliber rollingwith various reductions of area after performing a homogenizationtreatment by heating at 1200° C. and hot rolling at 1100° C. on a steelincluding 0.6 wt % C, 18 wt % Mn, 1.5 wt % Al, residual Fe, andunavoidable impurities according to a manufacturing history of FIG. 1.

An initial tensile curve and an optical micrograph of the hot rolledsteel were shown in FIGS. 2( a) and 2(b), respectively. FIG. 2( a) showsan initial tensile curve of the hot rolled steel and it may beunderstood that the hot rolled steel had a yield strength of 309 MPa, atensile strength of 736 MPa, and an elongation of 60%. The foregoingsteel had high strength by means of the interaction betweentwins-dislocations through the formation of twins and has highelongation because the twins uniformly formed in the steel relieve localstress concentration sites. It may be confirmed in FIG. 2( b) that themicrostructure had equiaxed grains with annealing twins formed therein.

Meanwhile, FIG. 3 shows image maps of back scattered electrondiffraction patterns from the microstructures of (a) hot rolled steelhaving an initial grain size of 14.3 μm and cold caliber rolled steelwire rods having a reduction of area of (b) 31%, (c) 43%, and (d) 54%,respectively. From FIGS. 3( a) to 3(d), it may be observed that thestructure of mechanical twins changed from a single twin variant to fourtwin variants having <112> orientations in {111} planes as the reductionof area increases. The mechanical twins induced by deformation act asobstacles reducing a mean free path of dislocations and thus, increasethe strength of a material by acting as a strengthening mechanism havinga grain refinement effect on the steel wire rod.

Also, in order to investigate true tensile (compression) properties, thetrue tensile (compression) properties of the hot rolled steel and thecold caliber rolled steel wire rod having a reduction of area of 54%were evaluated. For the foregoing evaluation, tensile tests wereperformed with round tensile specimens having 3 mm diameters and 12.5 mmgauge lengths, and compression tests were performed by using cylindricalcompression specimens having 2.8 mm diameters and 4.2 mm lengths. Theresults thereof are presented in FIGS. 4( a) and 4(b), respectively.FIG. 4( a) was the result of the hot rolled steel and FIG. 4( b) was theresult of the cold caliber rolled steel wire rod having a reduction ofarea of 54%.

As shown in FIG. 4, both of the hot rolled steel and the cold caliberrolled steel wire rod had excellent compression properties. However,compression cracks were not generated and infinite compression strainwas obtained even though the cold caliber rolled steel wire rod having areduction of area of 54% was an ultra-high strength steel wire rodhaving a tensile strength of 1830 MPa. Excellent compression propertieswere shown because tensile dislocations and back stress formed duringcold caliber rolling exhibit the Bauschinger effect to obtain lowcompressive flow stress, and excellent elongation was obtained duringthe compression test because twins, which can be formed in twelve{111}<112> orientations, were formed only in four orientations duringtension and it left room for forming twins in other eight orientations.

Meanwhile, FIG. 5 shows tensile curves after heat treating the coldcaliber rolled steel wire rods having various reductions of area at 550°C. and 600° C. for 8 minutes. In FIG. 5, stress-relief heat treatmentswere applied to provide stabilities to the cold caliber rolled steelshaving low uniform strain ε_(peak) with respect to total strain ε_(tot).As shown in FIGS. 5( a) to 5(c), it may be confirmed that uniform strainwas increased without generating a decrease in strength when thestress-relief heat treatments were performed.

Manufacturing of bolt products using a typical steel wire rod hadlimitations of long product shipping time due to spheroidizing andquenching-tempering heat treatments and distortion of the bolt productsdue to the heat treatments. However, the steel wire rod of the inventionhas excellent cold heading quality so that a heat treatment may beomitted and thus, may be used as an innovative steel wire rod fornext-generation component and material industries.

Next, in order to evaluate hydrogen delayed fracture resistance of thesteel wire rod according to the invention, steel wire rods weremanufactured by performing cold caliber rolling after a homogenizationtreatment was performed by heating at 1200° C. and hot rolling wasperformed at 1100° C. on a steel including 0.6 wt % C, 18 wt % Mn, and1.5 wt % Al in Fe according to the manufacturing history of FIG. 1.

According to the reduction rates during cold caliber rolling of thesteel wire rod, a sample caliber rolled at a reduction of area of 82%(Inventive Example 1), a sample caliber rolled at a reduction of area of70% (Inventive Example 2), a sample caliber rolled at a reduction ofarea of 54% (Inventive Example 3), a sample caliber rolled at areduction of area of 43% (Inventive Example 4), and a sample caliberrolled at a reduction of area of 31% (Inventive Example 5) wereprepared, respectively.

Meanwhile, a sample having a tempered martensitic structure wasmanufactured as a Comparative Example of the invention by hot rollingand oil quenching after a heat treatment was performed at 900° C. for 30minutes without cold caliber rolling, and then air cooling after a heattreatment was performed at 460° C. for 90 minutes.

Also, a sample having a pearlite structure was manufactured as aComparative Example by heating at 1000° C. for 10 minutes after hotrolling, and then quenching to 520° C. through lead patenting and aircooling.

FIGS. 6( a) and 6(b) illustrate an image map and a grain map of backscattered electron diffraction patterns of Comparative Example and 6(c)and 6(d) illustrate an image map and a grain map of back scatteredelectron diffraction patterns of Inventive Example 3, respectively. Asshown in FIGS. 6( c) and 6(d), it may be confirmed in the invention thatmicrostructures having very dense mechanical twins were obtained byperforming cold caliber rolling. The deformation-induced mechanicaltwins act as obstacles decreasing a mean free path of dislocations andthus, increase the strength of a material by acting as a strengtheningmechanism having a grain refinement effect on the steel wire rod.

FIG. 8 is a graph illustrating changes in strength according to strainin Inventive Examples 1 to 5 and Conventional Example. From the resultsof FIG. 8, it may be understood that strength and ductility may bechanged according to the reduction of area during cold caliber rolling,and the strength may be further increased in comparison to ConventionalExample while the ductility is maintained through the cold caliberrolling.

Meanwhile, an anode hydrogen injection method and a low-speed tensiletest were performed in order to quantitatively compare hydrogen delayedfracture resistances and the results thereof are shown in FIG. 8.Different hydrogen injected aqueous solutions were used in order toinject the same amount of hydrogen into high manganese austenitic steelof Inventive Example 3, pearlitic steel of Comparative Example, andtempered martensitic steel of Conventional Example.

A hydrogen injection amount of Inventive Example 3 was about 100 timeslower than those of Comparative Example and Conventional Example in thesame environment because the trap sites of diffusible hydrogen inInventive Example 3 were smaller than those of Comparative Example andConventional Example. Therefore, hydrogen injection in Inventive Example3 was performed for three days at a current density range of 10 A/m² to50 A/m² in a more harsh aqueous solution, i.e., a mixed solution ofammonium thiocyanate and sodium chloride, and hydrogen injections inComparative Example and Conventional Example were respectively performedfor three days at a current density range of 1 A/m² to 20 A/m² in a lessharsh sodium chloride aqueous solution having a concentration of 0.1normal.

The low-speed tensile test was performed at a strain rate of 10⁻⁵/secand notch fracture strength was measured by simulating hydrogendiffusion towards a stress center region, and the results thereof arepresented in FIG. 9. As shown in FIG. 9, the larger the amount ofdiffusible hydrogen is, the lower the notch fracture strength is. Degreeof embrittlement by hydrogen is also an important point to be noted. Inthe case of Conventional Example and Comparative Example, the degrees ofhydrogen embrittlement (reduction of fracture strength in FIG. 9) wererespectively 68% and 58% with respect to about 2 ppm of diffusiblehydrogen, but the degree of hydrogen embrittlement was 14% in the caseof Inventive Example 3.

According to the invention, a steel wire rod having excellent coldheading quality and being enabled to omit a heat treatment as well asachieving high strength may be provided. Therefore, effects such asmanufacturing cost reduction, manufacturing defect reduction, andlightening of components using the steel wire rod may be obtained.

Also, the steel wire rod according to the invention has excellenthydrogen delayed fracture resistance despite ultra-high strength,thereby enabling to contribute to increase the durability and safety ofa component.

1. A method of manufacturing a steel wire rod having excellent coldheading quality and hydrogen delayed fracture resistance, comprising:heating a steel containing about 12 wt % to about 25 wt % of manganese(Mn) within a temperature range of about 1100° C. to about 1250° C.; hotrolling the heated steel within a temperature range of about 700° C. toabout 1100° C.; and cooling the hot rolled steel to a temperature of200° C. or less and cold caliber rolling or drawing to manufacture asteel wire rod.
 2. The method according to claim 1, wherein the steelcomprises about 12 wt % to about 25 wt % of manganese (Mn), about 0.5 wt% to about 1.0 wt % of carbon (C), about 1.0 wt % to about 2.0 wt % ofaluminum (Al), residual iron (Fe), and unavoidable impurities.
 3. Themethod according to claim 1, wherein the cold caliber rolling or drawingis performed at a reduction of area range of about 10% to about 70%. 4.The method according to claim 3, wherein the cold caliber rolling isperformed at a reduction of area range of about 30% to about 90%.
 5. Asteel wire rod having excellent cold heading quality and hydrogendelayed fracture resistance comprising about 12 wt % to about 25 wt % ofmanganese (Mn), wherein a microstructure of the steel wire rod comprisesa face centered cubic austenitic structure and a <112>{111} twin system.6. The steel wire rod having excellent cold heading quality and hydrogendelayed fracture resistance according to claim 5, wherein the steel wirerod comprises a total of twelve twin systems in four <112> orientationsand three {111} planes of a lattice and four twin variants are formed inone plane.
 7. The steel wire rod having excellent cold heading qualityand hydrogen delayed fracture resistance according to claim 5, wherein acomposition of the steel wire rod comprises about 12 wt % to about 25 wt% of manganese (Mn), about 0.5 wt % to about 1.0 wt % of carbon (C),about 1.0 wt % to about 2.0 wt % of aluminum (Al), residual iron (Fe),and unavoidable impurities.
 8. A method of manufacturing a bolt,comprising: heating a steel containing about 12 wt % to about 25 wt % ofmanganese (Mn) within a temperature range of about 1100° C. to about1250° C.; hot rolling the heated steel within a temperature range ofabout 700° C. to about 1100° C.; cooling the hot rolled steel to atemperature of 200° C. or less and cold caliber rolling or drawing tomanufacture a steel wire rod; cold heading the steel wire rod tomanufacture a bolt head; and performing a heat treatment on the coldforged steel wire rod within a temperature range of about 400° C. toabout 600° C.
 9. The method according to claim 8, wherein the steelcomprises about 12 wt % to about 25 wt % of manganese (Mn), about 0.5 wt% to about 1.0 wt % of carbon (C), about 1.0 wt % to about 2.0 wt % ofaluminum (Al), residual iron (Fe), and unavoidable impurities.
 10. Themethod according to claim 8, wherein the cold caliber rolling or drawingis performed at a reduction of area range of about 10% to about 70%. 11.The method according to claim 8, wherein the cold caliber rolling isperformed at a reduction of area range of about 30% to about 90%. 12.The method according to claim 8, wherein the heat treatment is performedfor about 10 minutes or less.